Electron lens for electron microscope and the like

ABSTRACT

Movable magnetic compensating pieces are disposed on a lower magnetic path unit of an electron lens. The compensating pieces are in partial contact with the lower magnetic path unit and moved toward and away from each other on the lower magnetic path unit by manipulation form the exterior of the lens.

United States Patent [151 3,643,191 Kasai et al. 1 Feb. 15, 1972 [54] ELECTRON LENS FOR ELECTRON [56] References Cited MICROSCOPE AND THE LIKE UNITED STATES PATENTS [72] Inventors: Shozo Kasai, Hachioji; Daisaburo Shimizu,

Katsuta; hi i Katagiri, Hachioji n of 2,586,559 2/1952 Page ..250/49.5 D UK Japan 2,637,000 4/1953 Page ..250/49.5 D UX [73] Assignee: Hitachi, Ltd., Tokyo, Japan Primary Emminer ceorge Harris [22] Filed: June 10, 1970 Attorney-Craig, Antonelli & Hill [21] Appl. No.: 44,935 [57] ABSTRACT [30] Ford A cation Prior Data Movable magnetic compensating pieces are disposed on a gn pp y lower magnetic path unit of an electron lens. The compensat- June 13, 1969 Japan ..44/55077 ing pieces are in partial contact with the lower magnetic path unit and moved toward and away from each other on the [52] US. Cl. ..335/212, 250/495 D lower magnetic path unit by manipulation form the exterior of [51] Int. Cl. ..H0li1/00 the lens. [58] Field of Search... ..335/2l0, 212, 298; 250/495 D 4 Claims, 6 Drawing Figures mtmamm 15 m2 SHEET 1 BF 2 k I A35 X WA 2 Q a v 5 c m n 3 A 2 m m 9 M\ 3 X y w M i w 3 F N INVENTORS DAISABURO smmzu SHOZO KASAI,

AND SHINTIRO KATAGIRI PATENTEBFEB 15 me SHDZO KASAI, DAISABURO SHIMIZU 'AND SHINII'RD KATAGIRI CW nnlanem, Sre nrk 1 Hill ATTORNEYS INVENTORS ELECTRON LENS FOR ELECTRON MICROSCOPE AND THE LIKE This invention relates to electron lenses for use in electron microscopes and the like, and more particularly to an electron lens which is provided with means for preventing undesirable deflection of the electron beam due to leakage flux produced by the lens itself or any other flux produced by other sources.

Leakage flux is produced more or less by electron lenses used in electron microscopes and the like, and especially the greatest amount of leakage flux is produced by the objective lens among the electron lenses. This leakage flux is objectionable in that the magnetic field of the lens is thereby distributed asymmetrically relative to the optical axis of the electron microscope, resulting in the presence of a magnetic field, that is, a deflecting magnetic field in a direction perpendicular to the optical axis. Since the electron beam is always subjected to deflection by this deflecting magnetic field, the following disadvantages result:

1. As is commonly known, the path of the electron beam must coincide with the optical axis. It is, however, ap-

parent that the above condition is not satisfied due to the presence of the deflecting magnetic field.

2. The leakage flux is increased and decreased depending on the variation in the exciting current supplied to the electron lens. Accordingly, the electron image of a specimen appearing on the fluorescent screen moves to and fro, each time the exciting current supplied to the electron lens is adjusted. i

In an attempt to overcome the above disadvantages, means such as a device for moving the electron gun in the horizontal direction has been provided so as to effect the so-called aligning manipulation for aligning the path of the electron beam with the optical axis. However, even with such aligning manipulation, it is difficult to sufficiently and satisfactorily align the path of the electron beam with the optical axis.

It has also been attempted to mechanically move the focusing lens thereby to focus the electron image at a predeter-' mined position on the fluorescent screen. However, it is extremely inconvenient to make the above manipulation each time the exciting currentsupplied to the electron lens is adjusted.

It is therefore an object of the present invention to provide an electron lens which is provided with compensating means for eliminating the deflecting magnetic field. I

Another object of the present invention is to simplify and facilitate the adjusting manipulation by the compensating means.

A further object of the present invention is to eliminate the need for mechanically moving the electron gun and the focusing lens in an electron microscope or the like by using an electron lens provided with the compensating means in such an electron microscope or the like.

In order to attain the above objects, the electron lens according to the present invention is characterized by the fact that the electron lens includes at least one movable magnetic compensating piece which is in partial contact with-a lower magnetic path unit and is moved relative to the lower magnetic path unit by manipulation from the exterior of the lens.

Other objects, features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic view showing the principle of the present invention; and

FIGS. 2 through 6 are schematic views showing preferred embodiments of the present invention.

Referring first to FIG. 1, an exciting coil l' is wound in such a manner as to define a hollow space therein and is covered on its outer peripheral surface with an iron cover I. The upper and lower end surfaces of the exciting coil 1' are covered with an upper magnetic path unit 2 and a lower magnetic path unit 3, respectively. An upper magnetic pole piece 4 and a lower magnetic pole piece 5 are disposed within the hollow space in the exciting coil 1, and a spacer 5' of antimagnetic material is disposed between these two pole pieces 4 and 5. The portion occupied by the spacer 5 alternately may be left in the form of an air gap. The electron lens having a structure as described above further includes a magnetic compensating piece 6 in the form of circular plate of ferromagnetic material in bodily contact with the lower magnetic path unit 3 and a plurality of adjusting screws 7 for causing movement of the magnetic compensating piece 6 relative to the lower magnetic path unit 3.

Suppose that the magnetic field of the lens is distributed symmetrically with respect to the optical axis X-X' when the magnetic compensating piece 6 is not provided, then the insertion of the magnetic compensating piece 6 in such a state of magnetic field distribution results in asymmetry of the magnetic field of the lens. In other words, a deflecting magnetic field (hereinafter to be referred to as a compensating magnetic field) appears due to the insertion of the magnetic compensating piece 6. The magnitude and the direction of this compensating magnetic field can be varied by suitably moving and adjusting the position of the magnetic compensating piece 6 by the adjusting screws 7 so that the compensating magnetic field cancels the deflecting magnetic field due to the leakage flux described previously. Thus, once the compensating magnetic field is adjusted to cancel the deflecting magnetic field, readjustment is unnecessary since these two fields vary in the same relation when the exciting current flowing through the coil 1 is increased'or decreased.

However, a problem arises from the fact that the magnetic compensating piece 6 is in bodily contact with the lower magnetic path unit 3. Since the magnetic compensating piece 6 is in bodily contact with the lower magnetic path unit 3, the flux B varies very little even with the movement of the magnetic compensating piece 6 and the moving distance of the magnetic compensating piece 6 is not entirely proportional to the amount of compensation by the magnetic compensating piece 6, the amount of compensation being abruptly increased as the magnetic compensating piece 6 approaches the pole piece 5. Therefore, the effectively compensatable range is extremely limited and this is impractical.

An embodiment of the present invention which overcomes the above problem is shown in FIG. 2, in which it will be seen that a magnetic compensating piece 8 is in partial contact with the lower magnetic path unit 3. By this arrangement, a part B of the flux B is shunted to the magnetic compensating piece 8 and flows into the lower magnetic path unit 3 again at a portion in the vicinity of the lower magnetic pole piece 5. Thus, the flux varies relatively greatly as the magnetic compensating piece 8 is moved to an adjusted position by the adjusting screws 7, and the amount of compensation is substantially proportional to the moving distance of the magnetic compensating piece 8 thereby widening the range of compensation.

In another embodiment of the present invention shown in FIG. 3, an airgap 9 is formed in the lower magnetic path unit 3 so that the majority of the flux passes through magnetic compensating pieces 10 thereby directly controlling the magnetic reluctance of the lens.

A further embodiment of the present invention shown in FIG. 4 is so constructed that a magnetic compensating piece 11 having such a structure as extending above and beneath the lower magnetic path unit 3 prevents outward flow of the leakage flux produced in the air gap of the lower magnetic path unit 3.

In another embodiment of the present invention shown in FIG. 5, a portion of the coil bobbin for the exciting coil 1 serves as a magnetic compensating piece. More precisely, the coil bobbin is composed of portions A and A made of ferromagnetic material and a portion A;, made of antimagnetic material. The portion A for example, may be in partial contact with the lower magnetic path unit 3 and is arranged to be moved to an adjusted position by the adjusting screws 7.

Still another embodiment of the present invention shown in FIG. 6 is so constructed that a part of the magnetic compensating piece 12 is inicontact with the lower magnetic path unit 3 adjacent to the lower magnetic pole piece 5.

It will be appreciated from the foregoing description that the present invention provides an electron lens which does not deflect the electron beam and has a widened range of magnetic field compensation by virtue of the provision of the magnetic compensating pieces.

Although only one magnetic compensating piece is used in the above embodiments, a plurality of the magnetic compensating pieces may be used.

We claim:

1. An electron lens for use in an electron microscope or similar apparatus comprising an exciting coil wound to define a hollow space therein, an iron cover covering the outer peripheral surface of said exciting coil, magnetic path units covering both end surfaces of said exciting coil, magnetic pole pieces disposed in spaced relationship within said hollow space of said exciting coil in contact with said magnetic path units, at least one magnetic compensating piece in partial contact with one of said magnetic path units and movable thereon, and means for moving the partial contact portion of said magnetic compensating piece on said magnetic path unit so as to adjust the reluctance of said magnetic path unit to compensate for the undesirable deflecting magneticfield magnetic path unit includes an annular airgap located between the areas of contact with said first and second flanges so that the majority of flux passes through said magnetic compensating piece.

4. An electron lens as defined in claim 3 wherein said disc is l-l-shaped in radial cross section so as to have legs which extend on either side of said one magnetic path unit and a central interconnecting member passing through said annular air- 

1. An electron lens for use in an electron microscope or similar apparatus comprising an exciting coil wound to define a hollow space therein, an iron cover covering the outer peripheral surface of said exciting coil, magnetic path units covering both end surfaces of said exciting coil, magnetic pole pieces disposed in spaced relationship within said hollow space of said exciting coil in contact with said magnetic path units, at least one magnetic compensating piece in partial contact with one of said magnetic path units and movable thereon, and means for moving the partial contact portion of said magnetic compensating piece on said magnetic path unit so as to adjust the reluctance of said magnetic path unit to compensate for the undesirable deflecting magnetic field produced by exciting said lens.
 2. An electron lens as defined in claim 1 wherein said magnetic compensating piece comprises a disc having a central aperture substantially coextensive with the hollow space in said coil and first and second flanges extending transverse to the plane of said disc surrounding said central aperture and the periphery of the disc, said compensating piece contacting said one magnetic path unit solely by way of said first and second flanges.
 3. An electron lens as defined in claim 2 wherein said one magnetic path unit includes an annular airgap located between the areas of contact with said first and second flanges so that the majority of flux passes through said magnetic compensating piece.
 4. An electron lens as defined in claim 3 wherein said disc is H-shaped in radial cross section so as to have legs which extend on either side of said one magnetic path unit and a central interconnecting member passing through said annular airgap. 